This optoelectronic “nose” (side and front views, left) and handheld reader-analyzer (right) detects TATP and many other compounds on the basis of the response of an array of colorimetric sensors embedded in a disposable cartridge.

Credit: Chem. Sci.

This optoelectronic “nose” (side and front views, left) and handheld reader-analyzer (right) detects TATP and many other compounds on the basis of the response of an array of colorimetric sensors embedded in a disposable cartridge.

Credit: Chem. Sci.

Triacetone triperoxide (TATP) is a highly unstable explosive prone to unintended detonation. Yet terrorists, such as those responsible for last week’s bombings in Brussels and the November 2015 attacks in Paris, are increasingly using the compound to inflict carnage.

Only 15 g of TATP is needed to utterly decimate this mailbox, as shown in this high-speed video.

Credit: Jimmie Oxley/URI

Despite its instability, TATP is attractive as a terror weapon because it is relatively easy to prepare and, until recently, was difficult to detect by standard explosives screening methods. Those methods, which are based on X-ray computed tomography (CT) and ion mobility spectrometry (IMS), have been modified and can now routinely detect TATP in airports and at other security checkpoints. Several other methods in various stages of commercialization, including some based on handheld scanners, can also detect TATP.

The problem that remains unsolved is figuring out how to stop terrorists from detonating explosives in crowded, unscreened public areas, such as subway stations and the unsecured sides of airports, which is where the terrorists in Brussels set off their bombs.

TATP has been implicated in several terrorist plots since 2001, when Richard Reid, the “shoe bomber,” tried to ignite a supply of the compound hidden in the soles of his shoes during a trans-Atlantic flight. In addition to the terror attacks in Brussels and Paris, the peroxide was also used in the 2005 London transit bombings.

Terrorists don’t need extensive chemistry training to prepare TATP. And the materials required to synthesize it—hydrogen peroxide, acetone, and mineral acid—are widely available in large quantities.

But unlike TNT and other common military explosives, “TATP is incredibly dangerous” even when it’s sitting on a shelf, says David A. Atkinson, who heads Pacific Northwest National Laboratory’s research in explosives and biological threat detection. “It can go off with the slightest shock or a bit of friction,” he notes. For that reason, TATP has typically been used in the past in small quantities as a detonator to trigger explosions of TNT or other stable explosives.

In the Brussels bombings, however, Belgian authorities reported that they recovered two undetonated bombs at the airport containing some 15 kg of TATP. The bombs, which were hidden in suitcases, also contained ammonium nitrate, metal bolts, and nails. Belgian officials reported that two additional bombs of similar description were found in a Brussels residence tied to the bombers.

Those bombs likely would have been detected had they been subjected to CT and IMS scanning. According to Jimmie C. Oxley, an explosives specialist at the University of Rhode Island (URI), manufacturers modified those instruments in response to terror plots in recent years to enable them to detect TATP.

By irradiating luggage with intense X-ray beams and measuring how much of the beam passes through an object, CT scanners probe the density and other properties of a bag’s contents. Automated algorithms then compare those data with a library of density values for a group of explosives that now includes TATP. Suspicious bags can be further inspected via IMS .

Oxley, who was reached by C&EN just after her team conducted a TATP test explosion at URI’s firing range, notes that commercial IMS instruments initially did not detect TATP. But they do now.

In IMS measurements, after an operator swabs a suspicious bag and inserts the swab into the instrument, a plume of sample ions enters a drift tube and interacts with a gas. Under the influence of an electric field, the ions are driven down the length of the tube, causing them to separate en route to a detector according to their mass, size, and shape. The method, which can detect picogram levels of various explosives in seconds, generates unique IMS signatures.

According to Oxley, the instruments were initially configured to detect negatively charged ions, such as the ones formed by common nitro-based explosives, but not positively charged ions, which TATP can form. Now the instruments, which are made by several manufacturers, including U.K.-based Smiths Detection and Morpho, a French company, quickly scan for both types of signals.

TATP can also be detected via fluorescence methods. Flir, a major instrument maker with headquarters in Wilsonville, Oregon, manufactures a handheld device, Fido X3, capable of detecting numerous explosives, including TATP. As an air sample is drawn into the device, explosive analyte molecules bind with specially designed conjugated aromatic polymers, causing a large change in a fluorescence signal.

Other TATP detection methods have been reported in recent years, and some of them are being commercialized. Trained dogs, for example, are often considered a gold standard in sniffing out explosives. But trainers have generally stayed away from teaching canines to find TATP because the compound is so unstable that they’ve feared harming the dogs and themselves. Oxley’s research group has developed a safe TATP canine training kit in which the explosive is encapsulated in an inert polymer and is in the process of commercializing it.

Along with a digital reader, the device contains a large array of dyes that change color upon reacting with explosives and other compounds. The reader compares the color patterns before and after the sensor array is exposed to a sample—a sniff of air, for example—and then generates a color difference map that serves as a chemical signature.

Recently, Suslick’s group demonstrated that the device can distinguish TATP from other peroxides and can distinguish one synthesis procedure from another by detecting residual impurities. That information, which can determine, for example, the type of acid used as a catalyst to synthesize TATP, may assist law enforcement in identifying the bomb maker (Chem. Comm. 2015, DOI: 10.1039/c5cc06221g).

The team also recently showed that this detection strategy works quickly and reliably in an inexpensive handheld prototype device that exhibits sensitivity to TATP in the low parts-per-billion range (Chem. Sci. 2015, DOI: 10.1039/c5sc02632f).

Palo-Alto-based iSense, a start-up company, is continuing to develop the optoelectronic nose technology for several applications, according to Sung H. Lim. Lim was a postdoctoral researcher with Suslick in 2006 and now serves as iSense’s chief technology officer.

Any of these detection methods—and others under development, including portable mass spectrometry techniques—can alert a security officer to the presence of TATP. As Oxley points out, TATP is fairly easy to detect because it has a relatively high vapor pressure and is therefore volatile. But these methods can succeed only if the source of the explosive—a suitcase, car, or terrorist’s contaminated clothing or hair—is screened.

“When one passenger at a time goes through an airport checkpoint, security has the chance to get up close and personal,” Atkinson says. “You don’t get the chance to sample that way out in crowded public places.”

Oxley proposes that one piece of the solution to this difficult security problem is using multiple small detectors to provide wide area surveillance in public places such as malls and arenas.

Suslick suggests that in some public places, operators could use small handheld units, such as his optoelectronic nose, for preliminary checks, in much the same way trained dogs are used to quickly buzz through long lines of people waiting to enter an event. He notes that, unlike dogs, electronic detectors don’t need to take breaks and aren’t easily distracted, for instance by little kids holding hotdogs.

Atkinson offers what’s likely to be a more controversial suggestion. “My approach would be to look for the bomb factories while the would-be perpetrators are honing their craft, instead of waiting for them to fill their backpacks and head out to do the deed.”

He proposes coupling intelligence information from wire taps, e-mail and chat room monitoring, and other surveillance with mobile chemical sensing. “It’s not normal for a guy who lives in an apartment to bring home large amounts of acetone." Maybe he’s just stripping a piece of furniture, Atkinson says. But maybe not. “If law enforcement learns of suspicious activities or odd smells, someone should do some chemical investigating.”

“This is a huge, serious threat,” Atkinson says, “but it’s not primarily a technology problem. It’s an operational problem. We have good methods for detecting explosives. But we need to be smarter about the way we use detection technology.”

Leave A Comment

*Required to comment

Paul E. Eckler (March 30, 2016 5:42 PM)

Control of these materials is difficult. Acetone is a common solvent used in many labs to clean glassware, but also as a paint thinner. Hydrogen peroxide 35% is available on Amazon. It is used in applications like hair bleach. Moreover, sodium percarbonate, which releases hydrogen peroxide in water, is present in many oxygen bleach products such as Oxi-Clean. Sodium percarbonate is also listed on Amazon. You would not be surprised to find both acetone and peroxide available in many drug stores and home improvements stores.

Leave A Comment

*Required to comment

Richard Aversa (April 7, 2016 2:25 PM)

I respectfully disagree. For example, if I need to remove a sharpie mark or the residue from a sticker, I should have the option of using acetone, isopropanol, or some other organic solvent. Just because I no longer work in a lab doesn't mean I shouldn't have access to useful reagents (within reason). There is always going to be somebody willing to misuse any commodity, and there are always going to be people who benefit from its proper use.

Leave A Comment

*Required to comment

Clifford Schoff (March 30, 2016 5:03 PM)

I was in Turkey at the time of the Ankara and Istanbul explosions. The Istanbul airport (both international and domestic sections) requires baggage to be x-rayed before the visitor can enter the building. I do not know if the x-ray operators are trained well enough to identify or suspect bombs, but the mere presence of security before entry may keep most bombers away.

Leave A Comment

*Required to comment

Rama Viswanathan (March 30, 2016 5:21 PM)

Excellent topical article on detection of explosives! I would add that Terahertz spectroscopic methods are also very promising and may circumvent some of the difficulties faced with other non-invasive methods, as described below at the web site of one of the leading groups (led by Tim Korter at Syracuse) researching this area (http://terahertz.syr.edu/?page_id=39).

Rama ViswanathanProfessor (emeritus) of Chemistry and Computer Education, Beloit College

Non-Invasive Explosives Detection and IdentificationThe detection and identification of concealed explosive threats is one of the greatest challenges facing the defense and security communities. There is a clear need for a technology that is able to address this problem in an accurate and safe manner. Terahertz radiation has attracted much attention as a potential solution. Terahertz radiation is able to pass through many common materials such as paper, plastic, and fabric much like X-rays, but unlike X-rays, THz radiation is non-ionizing and is safe to use in the direct screening of people. Also unlike X-ray methods, THz radiation is able to provide unambiguous chemical identification of concealed explosives through the interaction of the THz radiation with the unique crystalline lattice vibrations of these materials. The chemical specificity and non-invasive nature of THz spectroscopy enables the rapid discrimination of innocuous and hazardous materials (for example, two white powders such as sucrose vs. TNT) while minimizing the occurrence of false positives.

Leave A Comment

*Required to comment

Bill Hamm (March 30, 2016 5:21 PM)

This was exactly my thought when reading the article. I'm wondering about how to do this.One way is to restrict sales to companies that have some sort of verification, such as in hazardous waste disposal. To circumvent this, a terrorist could go to a lot of different drugstores and hardware stores, at least for acetone and hydrogen peroxide. Perhaps we need driver's licenses, such as is done for sudafed in the US. Globally is a much bigger problem, but I would think that this could easily be done in Europe and the US.

Leave A Comment

*Required to comment

Herbert Skovronek, Ph. D. (March 30, 2016 6:09 PM)

Gimme a break. It's 20+ years that everyone's been trying to replace the dog. But everyone takes the wrong approach and it ain't gonna happen. Dog gets tired? Use two overlapping dogs, success goes up from 97% to 97% X 97% or ~99.9+%.

Two suggestions that might help:1. all parcels entering ANY facility need to go through a "trap door" where the sides are brushed by a rotating belt on each side. These belts will pick up explosive traces for detection by any type of instrument.

2. Time to stop the nonsense. Public facilities (including airports, etc.) need to have tunnel approaches--WITH DETONATING CAPABILITY (electronic, heat, whatever). You walk through one at a time with your bag. End of question! This idea was presented to DARPA more than 20 yrs ago!

Leave A Comment

*Required to comment

Herb Skovronek, Ph. D. (not in statistics!) (March 31, 2016 9:52 PM)

Ram B: Please explain why 97% and 97% of the residual 3% doesn't give an overall 99.9%? What do you say it gives? 100 "issues", first dog finds 97% of the "issues"; the second dog searches the same 100 issues, finds 97%. Granted, they may find/miss the same 3 issues, but unlikely.hss

Leave A Comment

*Required to comment

Richard Wheeler (April 2, 2016 12:37 PM)

The proposal was to have "two overlapping dogs" because "Dogs get tired." In other words, when one dog gets tired, another dog takes its place. While the second dog is working, the first dog is recouping its energy.

Therefore, only one dog works at a time. When the first dog works, the hypothetical number is 97%. When the second dog works, the hypothetical number is still 97%.

97% plus (97% of the remaining 3%) characterizes a very different model in which two dogs work redundantly. It does not address the problem of fatigue because, assuming identical dogs, they both tire at the same time, and your sniffer system still goes offline. During that time, effectiveness drops to zero.

Other problems need consideration. There is a shortage of trained dogs and dog handlers, so costs are ballooning. When dogs become commodities, animal cruelty becomes rampant, especially in countries where dogs are either stoned or eaten. How many substances can a dog be trained to seek? Dogs don't just get tired, they get bored and are easily distracted, just like humans.

Leave A Comment

*Required to comment

Dr. Jon R. Askim (April 1, 2016 1:09 AM)

Dr. Skovronek,

There are many reasons to replace a dog, and olfactory fatigue is only one of them - honestly, though, it is not the primary one. Training a dog takes a significant amount of time, but also takes a highly trained and experienced operator. Once adequately developed, small devices and methods such as the ones presented above are cheap to produce, quick to implement, and require no additional training.

Transportation safety has experimented with many techniques for collecting and detecting explosives over the years, but these methods must strike a balance between travelers' rights to privacy as well as cost, speed, and accuracy. For your suggestions in particular, the "rotating belt" method that you described is very similar swabs of peoples' hands used by TSA agents currently - unless you are planning to replace this belt quite often (i.e., once per 5-10 pieces of luggage), cross-contamination would be a serious issue and could render the method ineffective. Additionally, attempting remote detonation on travelers' luggage would almost certainly damage this luggage in many cases - again, this chance of damage must be weighed against the need for public safety.

These are not simple issues, and it's why we continue to research non-invasive methods such as those discussed in this article. I assure you, too, that research on methods similar to the ones that you mentioned - yes, including dogs - is still being pursued vigorously across the country. This particular article is just highlighting some non-invasive methods, and I'm pleasantly surprised to see some of our research group's work discussed alongside the excellent work being done by PNNL, Jimmie Oxley, Flir, and Morpho.

Leave A Comment

*Required to comment

Peter Wepplo (March 30, 2016 6:15 PM)

I should hope that TTAP bombs should be easier to intercept simply due to more knowledge. It was reported the landlord and neighbors of the Brussels bombers noted a suspicious odor. Even the cab driver noted an odor as the would be bombers travelled to the airport. If the gendarmes could be equipped with one of these mobile devices, intercepting the production of these bombs might stop their production.

Leave A Comment

*Required to comment

Rolf E. Hummel, PHD (March 30, 2016 7:02 PM)

I am surprised that our 15+ year old explosion detection technique called the "differential reflectometry" is completely ignored. The technique is widely published and Oxley knows about us. It is currently further developed in Holland because there was no interest in it in the USA.

Leave A Comment

A. Crowdsource.Analytical ideas that increase monitoring and their logistics. These ideas then should be harvested for other applications like the space program .

B. Another crowd source idea in how to gather big data on possible common household chemicals leading to deep learning as to where these purchases could be directed for terrorist activity and still maintain privacy- a big challenge but still worth a criwdsource shot so to speak !!!C. Counter intuitive but develop secondary high school programs that educate our new generations as to the creative and extreme destructive power of chemistry

Leave A Comment

*Required to comment

Dr Antony Bigot (March 31, 2016 5:09 AM)

Why not, instead of trying to detect TATP, impairing its preparation by, lets say, adding traces of Fe or Cu salts in acetone. As a chemist, I would guess that those metals should decompose any peroxides as they are formed ...

Leave A Comment

*Required to comment

Suman Layek (March 31, 2016 6:48 PM)

Anytime you we put people through chicken coup, we will increase the chance of detonating a bomb within a crowd. Instead of creating a mile long line for security, create the security zone as scrambled as possible and put enough screeners to keep the lines short. This approach in a place like an airport. Open area, like what happened in Boston marathon can not be stopped with this approach. I don't think anything can save us from a monster who does care about his life.